a(n) = prime((a(n-1)+1)/2), a(1) = 9.

Terms

a(0) =9a(1) =11a(2) =13a(3) =17a(4) =23a(5) =37a(6) =67a(7) =139a(8) =349a(9) =1039a(10) =3727a(11) =16007a(12) =81853a(13) =492059a(14) =3436987a(15) =27605857a(16) =252391267a(17) =2602897093a(18) =30034790941a(19) =384953881369